1. Field of the Invention
This invention relates in general to submersible well pumps and in particular to a low volume submersible pump that has an impeller configuration designed for high specific head coefficient.
2. Description of the Prior Art
Submersible pumps of the type concerned herein are used in oil wells and in deep water wells. The submersible pump assembly includes an electric motor located in the well. The shaft of the motor extends through a seal section to drive a centrifugal pump. The pump delivers well fluid to the surface through tubing. A power supply at the surface is connected to the electrical pump motor for supplying power. In some installations, variable speed drive systems at the surface allow the operator to vary the frequency and voltage being supplied to the motor to vary the speed of the motor. Also, varying the speed to maintain a constant current and torque on the motor is not unknown.
Relatively low volume pumps, 50-300 barrels per day, are not as efficient as they could be theoretically. The impeller vanes can be oriented to provide theoretically a high stage head. However, although efficient, the pump would be unstable. The resulting shape of the pump output head versus flow rate curve would have a portion that is nearly horizontal. This horizontal portion would result in for a given head, more than one possible flow rate. Because of unknown well characteristics, the pump could shift between flow rates and even stop pumping, causing damage to the pump. As a result, artificial losses are built into these low volume pumps to create more of a downwardly sloping head/flow rate curve. While this stabilizes the pump, the pump will be less efficient and produce less head per stage.